<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    html {
      /* 根标签 */
      font-size: 20px;
    }

    .p1 {
      font-size: 10px;
    }

    .p2 {
      font-size: 20px;
    }

    .p3 {
      font-size: 30px;
    }

    #b1 p span {
      /* font-size: 2rem; */
      font-size: 200%;
    }
  </style>
</head>

<body id="b1">
  <p class="p1">this is p1 small
    <span>children</span>
  </p>
  <p class="p2">this is p2
    <span>children</span>
  </p>
  <p class="p3">this is p3
    <span>children</span>
  </p>

</body>

</html>